Hello
This might seem obvious.
You all know the dailies in Northrend, what you do is 25 dailies the day before launch, and then when cataclysm launches you can turn them in. It wouldn't take much time after that just using your flyingmount to Vash'ir or Mount Hyjal.
I recommend doing quests these places:
1. Argent Dawn Tournament (Icecrown)
2. Gunship Alliance / Horde (Icecrown)
3. Sons of Hodir camp (Storm Peaks)
You get around 22.000 xp. for every quest. Lets look here:
22.000 * 25 = 500.000 xp !
I don't know how much xp you need for level 81, but I guess It's around 2 million xp, and this gives it a huge boost.
I hope it could help you, even though it is kinda obvious